6 Host requirements (RFC1042) CSMA/CD Carrier sense multiple access with collision detection Send frame Retransmission counter N = 0 Every Internet host connected to 10 Mbit/s Ethernet should MUST be able send and receive packets using RFC894 (Ethernet) encapsulation. Should be able to receive RFC 1042 (IEEE 802) packets intermixed with RFC 894 packets. May be able send packets using RFC 1042 encapsulation. Link Layer - (c) Jouni Ikonen / Packet ready? Yes Channel busy? No Transmit Collision detected? No Done Link Layer - (c) Jouni Ikonen / No Yes Transmit jamming signal Increase retransmission counter, N = N +1 Delay R * 51.2 micro seconds Select random integer between R = [0, 2 K ] K = N K = 10 Yes N<=10 No Yes N <15 No Abort Collision detection Serial Interfaces Point in minimum frame size is that sending host realizes that collision has happened during it s transmission. If packet is short the sender might not detect a collision and believe that transmission was successful. Link Layer - (c) Jouni Ikonen / There are two specialized link layers for serial interfaces: SLIP (Serial Line IP) PPP (Point to Point Protocol) CSLIP and PPP can reduce number of bytes/frame transmitted over the link! Many header fields do not change. PPP is more modern and correct many deficiencies of SLIP. Link Layer - (c) Jouni Ikonen / SLIP : Serial Line IP SLIP is simple form of encapsulation for IP datagrams on serial lines Rules for the framing in SLIP : IP datagram is terminated by the special character called END (0xc0) if byte in datagram equals the END character, the 2- byte sequence 0xdb, 0xdc is transmitted instead 0xdb is called SLIP ESC character if byte in datagram equals the ESC character, the 2-byte sequence 0xdb, 0xdd is transmitted instead Link Layer - (c) Jouni Ikonen / Deficiencies in SLIP Each end must know the others IP address. There is no method for one end to inform other of its IP address. There is not type field. If a serial line is used for SLIP, it can t be used for some other protocol at the same time. There is no checksum added by SLIP. Upper layers must detect corrupted messages. Link Layer - (c) Jouni Ikonen / 2

8 Loopback key points Everything sent to the loopback address appears as IP input Datagrams sent to a broadcast address or a multicast address are copied to the loopback interface and sent out on the Ethernet. This is because the definition of broadcasting or multicasting includes the sending host Anything sent to one of the host s own IP MTU : Maximum Transmission Unit There is limit (MTU) on the size of the frame for both Ethernet encapsulation and encapsulation (1500 or 1492) IP performs fragmentation, breaking the datagram up into smaller pieces (fragments) so that every piece is smaller than the MTU Different types of networks have different sizes of MTUs address is sent to the loopback interface Link Layer - (c) Jouni Ikonen / Link Layer - (c) Jouni Ikonen / MTU When two hosts are communicating across multiple networks, each link can have a different MTU The smallest MTU between the two host is called the path MTU How about interaction? NETWORK MTU (bytes) Hyperchannel Mbits/sec token ring Mbits/sec token ring 4464 FDDI 4352 Ethernet 1500 IEEE 802.3/ X Point to point (low level) 296 Link Layer - (c) Jouni Ikonen / 4

10 IP header: TOS TOS => Diffserv 4 TOS bits are for: Minimize delay Maximize throughput Maximize reliability Minimize monetary cost Only one of 4 bits can be turned on. TOS is not supported by all (or most) of TCP/IP implementations. Some routing protocols like OSPF and IS-IS are capable of making routing decisions based on TOS. IP: Internet Protocol - (c) Jouni Ikonen / Lappeenranta univ. of Tech. Differientiated services field RFC 2474 The Addition of Explicit Congestion Notification (ECN) RFC New Terminology and Clarifications for Diffserv. RFC 3260 Renaming of TOS field to DS field IP: Internet Protocol - (c) Jouni Ikonen / Lappeenranta univ. of Tech. IP header: more fields Total length: Total length of IP datagram (=> max. IP datagram size is 2 16 ). Identification: uniquely identifies each datagram sent by a host. Normally incremented by one each time datagram is sent. Flags Do not fragment & more fragments TTL time-to-live: upper limit on number of routers which datagram can pass. Protocol: used for demultiplexing protocols What the heck is one s complement? IP header: checksum Header checksum is calculated over IP header. It does not include any data that follows the header. Sending host: Set checksum field to zero. Calculated as 16-bit one s complement sum of the header and places the result to the checksum field. Receiving end calculates the 16-bit one s complement sum of the header. If the result is a ones transmission is treated as successful. IP: Internet Protocol - (c) Jouni Ikonen / Lappeenranta univ. of Tech. IP: Internet Protocol - (c) Jouni Ikonen / Lappeenranta univ. of Tech. IP header: the options Options are rarely used and all hosts/routers do not support all the options. Options defined: Security and handling restrictions Record route Timestamp Loose source routing Strict source routing IP: Internet Protocol - (c) Jouni Ikonen / Lappeenranta univ. of Tech. IP Routing As a concept : Simple, if the destination is directly connected to a host (P2P, shared network) then the datagram is sent directly to the destination. Otherwise the datagram is send to a default router. IP Layer has a routing table in memory that it searches each time it receives a datagram to send IP: Internet Protocol - (c) Jouni Ikonen / Lappeenranta univ. of Tech. 2

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